1. Field of the Invention
The present invention relates to a projection exposure apparatus for use in a lithography step in the course of manufacturing a semiconductor element, a liquid crystal display element, etc.
2. Related Background Art
This kind of projection exposure apparatus has hitherto been classified roughly into two types. One of them may involve the use of a method of exposing a photosensitive substrate such as a wafer, a plate, etc. by a step-and-repeat method through a projection optical system having an exposure field capable of including a whole pattern of a mask (reticle). The other type may involve the use of a scan method of effecting the exposure with a relative scan performed under mask illumination of arched slit illumination light, wherein the mask and the photosensitive substrate are disposed in a face-to-face relationship with the projection optical system interposed therebetween.
A stepper adopting the former step-and-repeat exposure method is a dominant apparatus in the recent lithography process. The stepper exhibits a resolving power, an overlap accuracy and a throughput which are all higher than in an aligner adopting the latter scan exposure method. It is considered that the stepper will continue to be dominant for some period from now on into the future.
By the way, a new scan exposure method for attaining a high resolving power has recently been proposed as a step-and-scan method on pp.424-433 of Optical/Laser Microlithography II (1989), SPIE Vo1.1088. The step-and-scan method is a combined version of the scan method of one-dimensionally scanning the wafer at a speed synchronizing therewith while one-dimensionally scanning the mask (reticle) and a method of moving the wafer stepwise in a direction orthogonal to a scan-exposure direction.
FIG. 1 is an explanatory view showing a concept of the step & scan method. Herein, shot regions (one chip or multi-chips) arranged in an X-direction on a wafer W are scan-exposed by beams of arched slit illumination light RIL. The wafer W is stepped in a Y-direction. Referring to the same Figure, arrows indicated by broken lines represent a route of the step & scan (hereinafter abbreviated to S & S) exposure. The shot regions undergo the same S & S exposure in the sequence such as SA.sub.1, SA.sub.2, . . . SA.sub.6. Subsequently, the same S & S exposure is performed on the shot regions in the sequence such as SA.sub.7, SA.sub.8, . . . SA.sub.12 arranged in the Y-direction at the center of the wafer W. In the aligner based on the S & S method disclosed in the above-mentioned literature, an image of the reticle pattern illuminated with the arched slit illumination light RIL is formed on the wafer W via a 1/4 reduction projection optical system. Hence, an X-directional scan velocity of the reticle stage is accurately controlled to a value that is four times the X-directional scan velocity of the wafer stage. Further, the reason why the arched slit illumination light RIL is employed is to obtain such advantages that a variety of aberrations become substantially zero in a narrow (zonal) range of an image height point spaced a given distance apart from the optical axis by using a reduction system consisting of a combination of a refractive element and a reflex element as a projection optical system. One example of such a reflex reduction projection system is disclosed in, e.g., U.S. Pat. No. 4,747,678.
Proposed in, e.g., Japanese Patent Laid-open Application No. 2-229423 (U.S. Pat. No. 4,924,257) is an attempt to apply a typical projection optical system (full field type) having a circular image field to an S & S exposure method other than the above-described S & S exposure method which uses the arched slit illumination light. The following are particulars disclosed in this Patent Laid-open Application. Exposure light with which the reticle (mask) is illuminated takes a regular hexagon inscribed to a circular image field of a projection lens system. Two face-to-face edges of the regular hexagon extend in a direction orthogonal to the scan-exposure direction. It is thus attain the S & S exposure exhibiting a more improved throughput. That is, this Patent Laid-open Application shows that the scan velocities of the reticle stage and of the wafer stage can be set much higher than by the S & S exposure method using the arched slit illumination light by taking an as large reticle (mask) illumination region in the scan-exposure direction as possible.
According to the above-described prior art disclosed in Japanese Patent Laid-open Application No. 2-229423, the mask illumination region is enlarged in the scan-exposure direction to the greatest possible degree. This is therefore advantageous in terms of the throughput.
By the way, there is nothing but to take the zig-zag S & S method shown in FIG. 1 even in the apparatus disclosed in the above-mentioned Patent Laid-open Application in consideration of actual scan sequences of mask stage and the wafer stage.
The reason for this is Given as follows. A diameter of the wafer W is set to 150 mm (6 inch). When trying to complete the exposure of one-row shot regions corresponding to the wafer diameter by only one continuous X-directional scan, the premise is that a 1/5 projection lens system is employed. Based on this premise, a scan-directional (X-directional) length is as long as 750 mm (30 inch). It is extremely difficult to manufacture this kind of reticle. Even if such a reticle can be manufactured, a stroke of the reticle stage for scanning the reticle in the X-direction requires 750 mm or more. Therefore, the apparatus invariably highly increases in size. For this reason, there is no alternative but to perform the zig-zag scan even in the apparatus disclosed in the above-mentioned Patent Laid-open Application.
It is therefore required that the periphery of the pattern region on the reticle be widely covered with a light shielding substance so as not to transfer the reticle pattern within an adjacent shot region with respect to, e.g., the shot regions SA.sub.1, SA.sub.12 shown in FIG. 1.
FIGS. 2A and 2B each illustrate a layout of a hexagonal illumination region HIL, a circular image field IF of the projection lens system and a reticle R during a scan exposure. FIG. 2A shows a state where the hexagonal illumination region HIL is set in a start-of-scan position on the reticle R. Only the reticle R one-dimensionally moves rightward in the same Figure from this state. FIG. 2B illustrates a state at the end of one scanning process.
Referring to FIGS. 2A and 2B, the symbols CP.sub.1, CP.sub.2, . . . CP.sub.6 represent chip patterns formed in row in the X-direction on the reticle R. A row of these six chip patterns correspond to the shot regions to be exposed by one scanning process in the X-direction. Note that in the same Figures, the central point of the hexagonal illumination region HIL coincides substantially with the center of the image field, i.e., an optical axis AX of the projection lens system.
As obvious from FIGS. 2A and 2B, the light shielding substance equal to or larger than at least a scan-directional width dimension of the hexagonal illumination region HIL is needed for the exterior of the pattern region in the start- and end-of-scan areas on the reticle R. Simultaneously, a scan-directional dimension of the reticle R itself also increases. An X-directional moving stroke of the reticle stage is also needed corresponding to a total of an X-directional dimension of the entire patterns CP.sub.1 -CP.sub.6 and a scan-directional dimension of the hexagonal illumination region HIL. Those are thinkable problems in terms of shaping up an apparatus.